This invention relates to a color cathode-ray tube (CRT) and, more particularly to a color CRT having a tensioned focus mask.
A color cathode-ray tube (CRT) typically includes an electron gun for generating and directing three electron beams to a screen of the tube. The screen is located on an inner surface of the faceplate of the tube and comprises an array of elements of three different color-emitting phosphors. A color selection electrode, which may be either a shadow mask or a focus mask, is interposed between the gun and the screen, to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal that is contoured to somewhat parallel the inner surface of the tube faceplate. A focus mask comprises dual sets of parallel conductors that are perpendicular to each other and usually separated by an insulative layer. The first set of parallel conductors are spaced-apart metal strands in vertical orientation and the second set of parallel conductors are metal cross-wires in horizontal orientation. Each set of parallel conductors are electrically interconnected with the strands being held under tension. The focus mask structure forms substantially rectangular openings producing quadrupolar lenses when different voltages are applied to the two sets of parallel conductors.
The large voltage differences applied between the two sets of parallel conductors creates a susceptibility to surface flashover between the conductors. Surface flashover is a breakdown process that may take place on or near the surface of the insulating material separating the two sets of parallel conductors and may lead to arcing between the conductors at one or more places on the focus mask. Since the cross-wires, which form a set of parallel conductors, are electrically connected to one another, the entire mask becomes highly susceptible to this breakdown. The system can store a substantial amount of energy in the form of a capacitor. Consequently, individual junctions between the two sets of conductors are vulnerable to a rapid and substantially complete capacitive discharge of the entire tension focus mask. This stored energy discharge may be sufficient to cause local melting of the conductors and/or the insulating material and may result in an electrical short leading to the subsequent failure of the focus mask.
Thus, a need exists to electrically isolate the individual cross-wires from each other and their designated anode to overcome the above-mentioned drawbacks.
The present invention relates to a color cathode-ray tube having an evacuated envelope with an electron gun therein for generating at least one electron beam. The envelope further includes a faceplate panel having a luminescent screen with phosphor lines on an interior surface thereof. A tensioned focus mask, having a plurality of spaced-apart strands, is located adjacent to an effective picture area of the screen. The spacing between the strands defines a plurality of slots substantially parallel to the phosphor lines on the screen. Each of the strands has a substantially continuous insulating material layer formed on a screen-facing side thereof. A plurality of cross-wires are oriented substantially perpendicular to the plurality of strands and are bonded thereto by the insulating material layer. The cross-wires are electrically connected to each other and to a conductor by a resistive material supported by a busbar formed of an electrically insulating material. The resistive coating is a composite material comprising an electrical conductor and an oxide mixed with at least one silicate glass.